Electrochromic compositions based on diketopyrrolopyrroles

ABSTRACT

Compositions containing 
     (a) at least one 1,4-diketopyrrolo[3,4-c]pyrrole of the formula I, II, IIa, III or IV ##STR1##  and (b) an auxiliary redox system from the ferrocyanide, ferrocene or ammonium iron (II) sulfate series in combination with at least one conductive salt (c), where, in the formula I, 
     R and R 2  are, for example, --H and R 1  is --SO 3  Na, 
     in the formulae II and IIa, R and R 3  are, for example, --H, R 1  is --SO 3  H and p is 3, 
     in the formula III, R 4  and R 5  are, for example, phenyl and R 6  and R 7  are, for example, methyl, and 
     in the formula IV R 8  and R 9  are, for example, phenyl, 
     are suitable as electrochromic materials in display systems.

The present invention relates to novel compositions containing certaindiketopyrrolopyrroles in combination with an auxiliary redox system andat least one conductive salt, to the use of these compositions aselectrochromic materials in display systems, and to processes for theirpreparation.

Optical display systems for the formation of, for example, letters,numbers or images in display systems are known; they use, for example,liquid-crystal displays, electroluminescence displays or plasma displaysor electrochromic materials.

Electrochromic materials allow continuous and reversible changes in theabsorption and reflection of the optically effective compounds employedto be produced. With the aid of an electrochemical redox reaction, achange in colour is produced for a certain time on or very close to thedisplay electrode and repeated continuously at the desired intervalsdepending on the areas of application. Various electrochromic materialshave already been proposed: inorganic compounds, for example WO₃, V₂ O₅,iridium oxides, nickel oxides or Fe₄ [Fe(CN)₆ ]₃ (Prussian Blue), andfurthermore, organic materials, for example bipyridinium salts(so-called viologens), metal phthalocyanines of the rare earths,tris(bipyridyl)ruthenium(III) complexes or thieno[3,2-b]thiophenes, andpolymeric materials, such as polythiophenes or polypyrroles. In thisconnection, cf. Glastechnische Berichte 62/1989, No. 2, 38-45; Displays,October 1983, 221-225; Proceedings of the SID, Vol. 25/4(1984), 331-334;Japanese Journal of Applied Physics 26, 1352-1355 and 1356-1360; Angew.Chem., Int. Ed. Engl. 27 (1988), 560-561; Japanese Journal of AppliedPhysics 24, 1985, L178-L180.

The systems proposed therein allow single-coloured displays (for examplea specific display colour, in particular in inorganic materials) ormulticoloured displays (for example especially in organic materials) tobe produced; however, they do not always meet the high demands ofindustry, and in particular the wide range of customer wishes withrespect to the three primary colours, for example for television sets ordisplay systems, cannot always be achieved.

It has now been found that the use of certain diketopyrrolopyrroles incombination with an auxiliary redox system and at least one conductivesalt allows multicoloured, electrochromic displays of high contrast,long service life and good readability to be obtained.

The present invention accordingly provides compositions containing

(a) at least one 1,4-diketopyrrolo[3,4-c]pyrrole of the formula I, II,IIa, III or IV ##STR2## and (b) an auxiliary redox system from theferrocyanide, ferrocene or ammonium iron(II) sulfate series incombination with at least one conductive salt (c), where, in theformulae I, II and IIa, the two groups R, independently of one another,are --H, halogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ mercaptoalkyl,--CN, --CF₃ or phenyl, R₂ is --H or R₁, and R₃ is --H or a group of theformula --(CH₂)_(p) --R₁, p is an integer from 1 to 6, and R₁ is a groupof the formula --CO₂ L, --SO₃ L, --PO₃ L or --N⁺ (R₁₀)(R₁₁)(R₁₂)X⁻ inwhich X is a halogen ion, (SO₄)²⁻, (SO₃ OCH₃)⁻ or (SO₃ OC₂ H₅)⁻ and L is--H, a group of the formula ##EQU1## or ⁺ NH(R₁₀)(R₁₁)(R₁₂) where M is amonovalent, divalent or trivalent metal cation, n is the number 1, 2 or3, and R₁₀, R₁₁ and R₁₂, independently of one another, are --H, C₁ -C₁₈alkyl, C₇ -C₁₂ phenylalkyl, C₅ -C₆ cycloalkyl, or phenyl or naphthylwhich is unsubstituted or substituted by halogen, C₁ -C₄ alkyl or C₁ -C₄alkoxy, or R₁₀ and R₁₁, together with the nitrogen atom, form a5-6-membered heterocyclic radical, or R₁₀, R₁₁ and R₁₂, together withthe nitrogen atom, form a 5-6-membered aromatic heterocyclic radical, inthe formula III, R₄ and R₅, independently of one another, areunsubstituted or halogen-, C₁ -C₄ alkyl- or C₁ -C₄ alkoxy-substitutednaphthyl, phenyl or pyridyl, which in turn can be substituted by one ortwo halogen atoms, one or two C₁ -C₄ alkyl or C.sub. 1 -C₄ alkoxygroups, furthermore by C₁ -C₄ alkylmercapto, --CN, CF₃, --CCl₃, --CONH₂,C₂ -C₅ alkoxycarbonyl or unsubstituted or halogen-, CH₃ - or OCH₃-substituted phenyl, phenoxy, thiophenoxy or N-phenylcarbamoyl, and R₆and R₇, independently of one another, are C₁ -C₁₂ alkyl, C₁ -C₁₂ alkoxy,C₁ -C₁₂ alkylmercapto, C₁ -C₁₂ alkyl-C₂ -C₅ alkoxycarbonyl, C₁ -C₁₂alkyl-C₁ -C₄ -N-alkylcarbamoyl or unsubstituted or halogen-, C₁ -C₄alkyl-, C₁ -C₄ alkoxy-, C₁ -C₄ alkylmercapto-, CN- or CF₃ -substitutedphenyl, benzyl or benzoyl, and in the formula IV, R₈ and R₉,independently of one another, are C₁ -C₈ alkyl, C₃ -C₆ cycloalkyl, C₇-C₁₂ phenylalkyl, which may be substituted in the phenyl radical byhalogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ alkylmercapto, --CN,--CF.sub. 3, --CH₃, --CONH₂ or phenyl, or R₈ and R₉ are furthermoreunsubstituted or halogen-, C₁ -C₄ alkyl- or C₁ -C₄ alkoxy-substitutednaphthyl or unsubstituted or halogen-, C₁ -C₄ alkyl-, C₁ -C₄ alkoxy-, C₁-C₄ alkylmercapto-, --CN, --CF₃, --CCl₃, --CONH₂ or phenyl-substitutedphenyl, or are pyridyl.

The various alkyl groups in the above radicals may be branched orunbranched. C₁ -C₄ Alkyl is, for example, methyl, ethyl, n-propyl,isopropyl, n-butyl, sec-butyl or tert-butyl, but in particular methyl.C₁ -C₈ Alkyl may, for example, be as defined above for C₁ -C₄ alkyl oradditionally, for example, n-propyl, isopropyl, tert-pentyl, n-hexyl or1,1,3,3-tetramethylbutyl. C₁ -C₁₂ Alkyl and C₁ -C₁₈ alkyl are, forexample, as defined above, or furthermore n-heptyl, n-octyl, nonyl,decyl, undecyl or dodecyl, or tridecyl, tetradecyl, pentadecyl,hexadecyl, heptadecyl or octadecyl.

Examples of C₁ -C₄ alkoxy and C₁ -C₁₂ alkoxy are methoxy, ethoxy,n-propoxy, isopropoxy, n-butoxy, sec-butoxy, tert-butoxy, n-pentoxy,isopentoxy, n-hexyloxy, octyloxy, decyloxy and dodecyloxy, but inparticular methoxy.

Examples of C₁ -C₄ alkylmercapto and C₁ -C₁₂ alkylmercapto aremethylmercapto, ethylmercapto, n-propylmercapto and isopropylmercapto,and n-hexylmercapto, octylmercapto and dodecylmercapto.

In the various definitions, halogen is, for example, fluorine, chlorineor bromine, but in particular chlorine.

A halogen ion X.sup.⊖ is, for example, Br⁻, F⁻, I⁻ or, in particular,Cl⁻.

C₃ -C₆ Cycloalkyl and C₅ -C₆ cycloalkyl are, for example, cyclopropyl orcyclobutyl, and cyclopentyl or cyclohexyl.

C₇ -C₁₂ Phenylalkyl is, for example, benzyl or phenylethyl.

C₂ -C₅ Alkoxycarbonyl is, for example, methoxycarbonyl, ethoxycarbonyl,n-propoxycarbonyl or butoxycarbonyl.

R₄ and R₅, R₆ and R₇, and R₈ and R₉ are preferably identicalsubstituents.

If R₁₀ and R₁₁ together with the nitrogen atom to which they are bonded,form a 5-6-membered heterocyclic radical, it is, for example, apyrrolidine, morpholine or piperidine radical.

If R₁₀, R₁₁ and R₁₂ together with the nitrogen atom to which they arebonded, form a 5-6-membered aromatic heterocyclic radical, this is, forexample, a pyrrole, pyridine, picoline, pyrazine, quinoline orisoquinoline radical.

Examples which may be mentioned of ⁺ NH(R₁₀)(R₁₁)(R₁₂) in the definitionof L are: ⁺ NH₄, ⁺ NH₃ CH₃, ⁺ NH₂ (CH₃)₂, ⁺ NH₃ C₂ H₅, ⁺ NH₂ (C₂ H₅)₂, ⁺NH₃ isopropyl, ⁺ NH₃ cyclohexyl, ⁺ NH₂ (CH₃)(C₆ H₅), ⁺ NH₃ C₆ H₅, ⁺ NH₃(para-tolyl), ⁺ NH₃ benzyl, ##STR3##

Examples of --N⁺ (R₁₀)(R₁₁)(R₁₂)X⁻ are --NH₃ ⁺ Cl⁻, --NH₂ (CH₃)⁺ Cl⁻,--NH(CH₃)₂ ⁺ Cl⁻, --NH₂ (C₂ H₅)⁺ Cl⁻, --NH₂ (C₆ H₅)⁺ Cl⁻ and --NH₂ (CH₂C₆ H₅)⁺ Cl⁻.

If L is a group of the formula ##EQU2## it is an alkali metal cation, analkaline earth metal cation or a transition metal cation, but inparticular Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Mn²⁺, Zn²⁺, Cu²⁺, Ni²⁺,Cd²⁺, Co³⁺, Al³⁺ and Cr³⁺ ; n is in particular 1 or 2, but veryparticularly 1; M is preferably Na⁺ ; and p is preferably 1, 2 or 3.

Examples of C₁ -C₁₂ alkyl-C₂ -C₅ alkylcarbonyl and -C₁ -C₄ carbamoyl are--CH₂ COOCH₃, --CH₂ COOC₂ H₅, --CH₂ COOC₄ H₉, --CH₂ CH₂ COOC₂ H₅, --CH₂CH₂ CH₂ COOC₂ H₅, and --CH₂ CONH₂, --CH₂ CH₂ CONH₂ and --CH₂ CH₂ CH₂CONH₂.

Pyridyl is 2-, 3- or preferably 4-pyridyl.

Preferred compositions preferably contain a compound of the formula I,II, IIa, III or IV in which, in the formulae I, II and IIa, the twogroups R, independently of one another, are --H, --Cl, --CH₃ or --OCH₃,R₂ is --H or R₁ and R₃ is --H or a group of the formula --(CH₂)_(p)--R₁, p is an integer from 1 to 6, and R₁ is a group of the formula--CO₂ L₁ or --SO₃ L₁ where L₁ is --H, Na or K, in the formula III, R₄and R₅, independently of one another, are phenyl which is unsubstitutedor substituted by one or two chlorine atoms, one or two C₁ -C₄ alkyl orC₁ -C₄ alkoxy groups, methylmercapto, --CN, phenyl, phenoxy orthiophenoxy, and, in the formula IV, R₈ and R₉ are 4-pyridyl, or phenylwhich is unsubstituted or substituted by --Cl, --Br or by one or two CH₃or OCH₃ groups.

Of very particular interest are compounds of the formula I or IV inwhich R is --H, R₂ is --H or R₁ and R₁ is a group of the formula --SO₃L₁ where L₁ is Na or K, and R₈ and R₉ are phenyl, 3- or 4-chlorophenyl,3,4-dimethoxyphenyl or 3- or 4-methoxyphenyl.

The diketopyrrolopyrroles of the formulae III and IV are known productsand can be prepared, for example, as described in U.S. Pat. Nos.4,579,949, 4,659,775 and 4,778,899 (formula IV) and 4,585,878 and4,666,485 (formula III).

With the exception of the compounds of the formula I in which R₁ is agroup of the formula --N⁺ (R₁₀)(R₁₁)(R₁₂)X⁻, the compounds of theformula I are known products and can be prepared, for example, asdescribed in U.S. Pat. No. 4,791,204.

By contrast, the compounds of the formula I in which R₁ is a group ofthe formula --N⁺ (R₁₀)(R₁₁)(R₁₂)X⁻ and the compounds of the formulae IIand IIa are novel and are therefore a further subject-matter of theinvention, the symbols R, R₁, R₂, R₃, p, L, X, M, n, R₁₀, R₁₁ and R₁₂given therein being defined as above.

The compounds of the formula II can be prepared by processes which areknown per se, for example by mono- or dicondensation of a compound ofthe formula ##STR4## with a compound containing the radical --(CH₂)_(p)--R₁ as leaving group (for example ethyl chloroacetate) in a suitableorganic solvent.

The compounds of the formula IIa can be prepared, for example, byreacting 2 mol of an appropriately substituted aryl nitrile of theformula ##STR5## or a mixture of these compounds with 1 mol of asuccinic acid diester, for example the dimethyl ester, in an analogousmanner to that described in U.S. Pat. No. 4,579,949.

The compound of the formula I in which R₁ is a group of the formula --N⁺(R₁₀)(R₁₁)(R₁₂)X⁻ can be prepared, for example, by quaternising acompound of the formula ##STR6## by processes which are known per se,for example using dimethyl sulfate or diethyl sulfate.

The compositions according to the invention can be obtained, forexample, by mixing the individual components (a) and (b) in the desiredmixing ratio. However, they can also be dissolved individually ortogether in a solvent which is suitable according to the invention, andthe resultant solution can be employed directly for the desiredapplication.

The mixing ratios of component (a) and the auxiliary redox system (b)may vary. However, molar amounts of these two components are expedientlyemployed.

The auxiliary redox systems (b) used according to the invention areknown compounds. They are expediently employed in pure form. Components(a) of the compositions according to the invention are expediently alsoemployed in pure form.

An example of ammonium iron(II) sulfates is (NH₃)₂ Fe(SO₄)₂.6H₂ O.

Examples of ferrocyanides which are suitable according to the inventionare alkali metal salts, alkaline earth metal salts or ammonium saltsthereof, for example sodium ferrocyanide, potassium ferrocyanide,calcium ferrocyanide, magnesium ferrocyanide or ammonium ferrocyanide.However, potassium ferrocyanide and sodium ferrocyanide are preferred.

The concentration of the auxiliary redox system (b) can be varied asdesired; it may be, for example, between 0.001 and 0.1M, but expedientlybetween 0.01 and 0.02M, based on the diketopyrrolopyrrole component (a).The concentration is frequently determined by the solubility of thediketopyrrolopyrroles which are suitable according to the invention,which, depending on the chemical structure, have a quite differentsolubility in the application medium. Equimolar amounts of components(a) and the redox system (b) are preferred.

In principle, the choice of redox system (b) is not crucial. However, aredox system which is soluble in the application medium and whose redoxpotential is as close as possible to that of the chosen component (a) isexpedient. In these cases, the process potential applied can be keptvery low.

The ferrocyanide salts are expediently employed in aqueous systems or inmixtures of water with organic water-miscible or partially misciblesolvents. By contrast, ferrocene is in particular used in organicsystems, although small amounts of water can also be used as long asthis compound remains dissolved.

Conductive salts (c) which are suitable according to the invention areall salts which are conventionally employed in voltametry and are ableto generate anions and cations in the application medium, for example:

cations: Li⁺, Na⁺, K⁺, N⁺ (C_(n) H_(2n+1))₄, where n is 1, 2, 3 or 4;

anions: Cl⁻, Br⁻, I⁻, ClO₄ ⁻, BF₄ ⁻, SCN⁻, CF₃ SO₃ ⁻, H₂ PO₂ ⁻, HPO₃ ²⁻,H₂ PO₄ ⁻ and HCO₃ ⁻.

Alkali metal halides or tetraalkylammonium halides, fluoroborates orperchlorates are expediently employed.

When selecting the conductive salt, it must be ensured that it is inertand only increases the conductivity in the application medium. Theconcentration of the conductive salt can be varied, but is expedientlybetween 3 and 100 times, preferably between 10 and 80 times, inparticular about 50 times, higher than that of the pyrrolopyrrolecomponent (a).

As stated above, the compositions according to the invention aresuitable as electrochromic materials in various display systems. To thisend, they can be used directly in solution in aqueous or organicsystems, or mixtures of water with an organic solvent.

For the aqueous systems, water of the highest possible purity, forexample demineralised water, is expediently used, so that theconcentration of the conductive salt or salts can be adjusted accuratelyor monitored during application and side reactions at the electrodes (inparticular at the counterelectrode) can be avoided.

Organic systems are inert solvents, preferably water-miscible orpartially miscible solvents, for example aliphatic alcohols containing 1to 5 carbon atoms, such as methanol, ethanol, propanol, isopropanol,n-butanol, 2-butanol, tert-butanol and n-pentanol, ketones, such asacetone, methyl ethyl ketone or methyl isobutyl ketone, glycols, such asethylene glycol, ethylene glycol monomethyl and monoethyl ether ordiethylene glycol, dioxane, tetrahydrofuran, and furthermore amides,such as dimethylformamide, dimethylacetamide, pyrrolidone orN-Methyl-2-pyrrolidone (NMP), acetonitrile, tetramethylurea,N,N'-dimethylpropyleneurea (DMPU), 1,2-dimethyl-2-imidazolidinone(DMEU), N,N,N',N'-tetraethylsulfamide (TES), dimethyl sulfoxide,dimethyl sulfone, tetramethyl sulfone, hexamethylphosphoric triamide,N,N,N',N'-tetramethylmethylphosphonic diamide, ethylene carbonate,ethylene sulfite, N,N,N',N'-tetramethylethylenediamine and1,2-dimethylethane.

Mixtures of the abovementioned solvents with water are also suitable.

The concentration of component (a) of the compositions according to theinvention in the abovementioned solvent is primarily determined by itssolubility (it must be in solution). In general, the concentration isbetween 0.001 and 0.5 mol, in particular between 0.01 and 0.02 mol.

For the aqueous or partially aqueous systems, diketopyrrolopyrroles ofthe formula I or II which contain water-solubilising groups areexpediently used. For the other systems, by contrast,diketopyrrolopyrroles of the formula III or IV are preferred.

As stated above, the compositions according to the invention can be usedas electrochromic materials in various display systems, thepyrrolopyrrole components, interacting with their reduced form,electrochemically modifying their optical behaviour (reflection ortransparency) in the application medium in combination with theauxiliary redox system employed and the conductive salt in such a mannerthat a visual and reversible change in colour can be produced.

Various systems or apparatuses can be used for this purpose, for exampleas in "Displays", October 1983, 221-225, or Glastechnische Ber. 62(1989), 38-45. A solution of a composition according to the invention ina suitable cell is subjected for a certain time to a certain directvoltage between two electrodes (the so-called display electrode and thecounterelectrode) so that the pyrrolopyrrole component (colourless orslightly yellowish in dissolved form) is reduced electrolytically on orvery close to the display electrode, and the resultant product becomescoloured, the colour change produced in this way corresponding to thedesired form at the display electrode employed. After reversal of thedirect voltage, the colour change produced in this way disappears, andthe process can be repeated as desired.

Suitable display electrodes are transparent or nonopaque electrodes, forexample made from indium oxide (In₂ O₃), indium-tin oxides, zinc oxide,titanium dioxide, antimony tin oxide (NESA), arsenic tin oxide, copperiodide, cadmium tin oxide (Cd₂ SnO₄) and in particular indium and tinmixed oxides (so-called ITO electrodes). Electrodes of this type areexpediently in the form of a coating on a glass substrate. Furthersuitable electrodes are very thin metal films.

Suitable counterelectrodes, where a corresponding electrochemicalreaction takes place, are also expediently transparent electrodes of theabovementioned composition. However, it is also possible to use otherelectrodes, for example coloured electrodes. In this case, a membranewhich is permeable to the ions suitable according to the invention mustbe installed in the solution according to the application between thetwo electrodes in order to screen the inherent colour of this electrode,so that light reflection from the coloured counterelectrodes onto thedisplay electrode can be excluded.

Expediently, low direct voltages are applied between the two electrodes,for example direct voltages of between 0.1 and 10 V, preferably between0.2 and 5 V, very particularly between 0.5 and 2.0 V.

The shape of the displays, in particular for numbers, can be controlledor formed, for example, by using a 7-segment device by suitablecontrolled switching of the electrodes in accordance with the shape ofthe desired numbers.

A system which is preferred according to the invention contains adiketopyrrolopyrrole of the formula IV where R₈ and R₉ are phenyl, incombination with ferrocene as the auxiliary redox system in a dimethylsulfoxide solution and with an ITO display electrode.

For aqueous systems, it may be particularly advantageous to usewater-soluble porous polymers, for example agar, gelatin,methylcellulose, polyvinyl alcohol or polyvinylpyrrolidones, in aconcentration of, for example, from 0.1 to 10% by weight, based on theamount of solvent or water. The polymer is expediently added in order tofix the electrochemically modified coloured component (a) on or in theregion of the display electrode. This makes it possible to prevent thecoloured component (a) moving too far from the electrode and resultingin excessively diffuse or unsharp images.

The compositions according to the invention can be used to obtainvarious coloured displays, for example from orange via red to violetdisplay colours on a colourless background.

The compositions according to the invention can be used in variousdisplay systems, for example for the formation of letters, numbers andimages in watches, display panels and display instruments, furthermorein X/Y matrix displays for screens, for example for TV sets, based onelectrochromic displays. Furthermore, the systems according to theinvention can also be used for smart windows (function-modifiablewindows). The displays obtained are multicoloured, easily read,irrespective of the viewing angle, and have good contrast. The memoryeffect associated therewith can also be utilised according to theapplication. In addition, various types of display can be produced usingsmall direct voltages.

The examples below illustrate the invention.

EXAMPLE 1

A 1 mM DMSO solution of a compound of the formula IV in which R₈ and R₉are phenyl, 1 mM of ferrocene and 50 mM of tetrabutylammoniumperchlorate is introduced into a cell containing two ITO electrodes, asdescribed in Japanese Journal of Applied Physics 26, p. 1357, FIG. 2. Achequered, red image pattern 1.0 cm² in size is produced at the displayelectrode by applying a direct voltage of 2 V for one second. A contrastratio of 10 at 585 nm is achieved, this number indicating the ratiobetween the transmission of the base solution (at λ585) and thetransmission of the coloured display electrode at the same wavelength.Using this system, up to 10⁵ write/erase cycles can be obtained.(DMSO=dimethyl sulfoxide).

EXAMPLE 2

Example 1 is repeated, but the ferrocene is replaced by the same amountof sodium ferrocyanide (1 mM) and in addition 10% by weight ofdemineralised water are used in the DMSO solution, and a direct voltageof 1.0 V is applied instead of 2 V. The same contrast ratio as above isobtained, and up to 10⁴ cycles are achieved.

EXAMPLE 3

A 0.25 mM NMP solution of a compound of the formula IV in which R₈ andR₉ are meta-chlorophenyl, 2 mM of ferrocene and 100 mM of sodiumperchlorate is introduced into the same cell as in Example 1. A reddisplay is produced by applying a direct voltage of 2.0 V for onesecond, and 10⁴ write/erase cycles (one second between each writing anderasing operation) are obtained. Constrast ratio at 580 nm: 4;(NMP=N-methyl-2-pyrrolidone).

EXAMPLE 4

Example 1 is repeated, but with the difference that the compound of theformula IV indicated therein is replaced by 10 mM of a pyrrolopyrrolecompound of the same formula, in which R₈ and R₉ are para-tert-butyl, 5mM of ferrocene (instead of 1 mM) and 150 mM of tetrabutylammoniumperchlorate are used (instead of 50 mM). This electrochromic systemgives a contrast ratio of 20 and a life of 10⁵ cycles.

EXAMPLE 5

30 mM of a compound of the formula IV in which R₈ and R₉ are3,4-dimethoxyphenyl, 15 mM of potassium ferrocyanide and 200 mM ofsodium perchlorate are dissolved in a mixture of 90% by weight ofdioxane and 10% by weight of water, and the solution is treated furtheras in Example 1, with a direct voltage of 1.1 V being appliedalternately for one second (the time without voltage is also onesecond). This system gives a contrast ratio of 33 and a life of 10⁵cycles.

EXAMPLE 6

The procedure is as in Example 5 above, but the pyrrolopyrrole indicatedtherein is replaced by 30 mM of a compound of the formula IV in which R₈and R₉ are para-isopropylphenyl. Equally good displays are obtained.

EXAMPLE 7

A 0.02M aqueous solution of a compound of the formula I in which R is--H and R₁ and R₂ are para-SO₃ Na, 0.02M of sodium ferrocyanide and 0.5Mof potassium chloride is treated with a direct voltage of 1.5 V asdescribed in Example 1 (writing duration: 1 second, erasing time: 1second). A contrast ratio of 28 at 560 nm and a life of more than 10⁴cycles are obtained.

EXAMPLE 8

The procedure is as in Example 7 above, but the pyrrolopyrrole indicatedtherein is replaced by a compound of the formula I in which R is --H andR₁ and R₂ are para-COONa. A contrast ratio of 8 at 560 nm and a life of10³ cycles are obtained.

EXAMPLE 9

The procedure is as in Example 7 above, but an additional 5% by weightof gelatin and a voltage of 1.6 V (instead of 1.5 V) are used. Acontrast ratio of 20 and a life of more than 10³ cycles are obtained.

EXAMPLE 10

An aqueous solution containing 0.01M of the pyrrolopyrrole compounddescribed in Example 7, 6% by weight of polyvinyl alcohol, 0.01M ofpotassium ferrocyanide and 0.2M of tetrabutylammonium perchlorate istreated at 1.8 V in the same way as in Example 1 (writing time 1 second,erasing time 1 second). A contrast ratio of 15 and a life of more than10³ cycles are obtained.

What is claimed is:
 1. A compound of formula I ##STR7## wherein the twogroups R, independently of one another, are --H, halogen, C₁ -C₄ alkyl,C₁ -C₄ alkoxy, C₁ -C₄ mercaptoalkyl, --CN, --CF₃ or phenyl,R₂ is --H orR₁, and R₁ is a group of the formula --N⁺ (R₁₀)(R₁₁)(R₁₂)X⁻ in which Xis a halogen ion, (SO₄)⁼, (SO₃ OCH₃)⁻ or (SO₃ OC₂ H₅)⁻, and R₁₀, R₁₁ andR₁₂, independently of one another, are --H, C₁ -C₁₈ alkyl, C₇ -C₁₂phenylalkyl, C₅ -C₆ cycloalkyl, phenyl, naphthyl, or said phenyl or saidnaphthyl substituted by halogen, by C₁ -C₄ alkyl or by C₁ -C₄ alkoxy; orR₁₀ and R₁₁, together with the nitrogen atom to which they are bonded,form a 5-6-membered heterocyclic radical which is pyrrolidino,morpholino or piperidino; or R₁₀, R₁₁ and R₁₂, together with thenitrogen atom to which they are bonded, form a 5-6-membered aromaticheterocyclic radical which is a quaternized pyrrolyl, pyridyl, picolyl,pyrazinyl, quinolyl or isoquinolyl group.
 2. A compound of formula II orIIa ##STR8## wherein the two groups R, independently of one another, are--H, halogen, C₁ -C₄ alkyl, C₁ -C₄ alkoxy, C₁ -C₄ mercaptoalkyl, --CN,--CF₃ or phenyl,R₃ is --H or a group of the formula --(CH₂)_(p) --R₁, pis an integer from 1 to 6, R₁ is a group of the formula --CO₂ L, --SO₃L, --PO₃ L or --N⁺ (R₁₀)(R₁₁) (R₁₂)X⁻ in which X is a halogen ion,(SO₄)²⁻, (SO₃ OCH₃)⁻ or (SO₃ OC₂ H₅)⁻ and L is --H, a group of theformula M^(+n) /n or ⁺ NH(R₁₀)(R₁₁)(R₁₂) where M is a monovalent,divalent or trivalent metal cation, n is the number 1, 2 or 3, and R₁₀,R₁₁ and R₁₂, independently of one another, are --H, C₁ -C₁₈ alkyl, C₇-C₁₂ phenylalkyl, C₅ -C₆ cycloalkyl, or phenyl or naphthyl which isunsubstituted or substituted by halogen, C₁ -C₄ alkyl or C₁ -C₄ alkoxy,or R₁₀ and R₁₁, together with the nitrogen atom to which they arebonded, form a 5-6-membered heterocyclic radical which is pyrrolidino,morpholino or piperidino; or R₁₀, R₁₁ and R₁₂, together with thenitrogen atom to which they are bonded, for a 5-6-membered aromaticheterocyclic radical which is a quaternized pyrrolyl, pyridyl, picolyl,pyrazinyl, quinolyl or isoquinolyl group.